1. Field of the Invention
This invention relates to search and retrieval of foreign objects from a heat exchanger and more particularly to independently positioning multiple search and retrieval devices within a nuclear steam generator.
2. Background of Information
In a pressurized water nuclear powered electric generating system, the heat generated by a nuclear reactor is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. Such a generator typically comprises an outer vertically oriented shell, a horizontal plate called a tube sheet adjacent to the lower end of the shell, a bundle of vertical U-shaped tubes supported by the tube sheet, and a wrapper barrel inside of the outer shell surrounding the tubes and extending from the upper portion of the shell downwardly to a predetermined point above the tube sheet. The wrapper barrel forms a narrow annulus inside the shell and generally extends down to a point approximately twelve to fourteen inches above the tube sheet. The outer cylindrical shell is provided with one or more openings of limited size called handholes which are typically located about five to twenty-one inches above the tube sheet. These handhole openings are covered during operation of the steam generator but may be opened when the generator is shut down to permit access to the area inside for maintenance purposes.
Occasionally, during maintenance inside the steam generator, objects such as bolts, wires or other foreign objects are inadvertently introduced into the system and have to be removed. Due to the limited space within the generator, the annulus between the wrapper barrel and the shell generally is only about 1.5 to 2.75 inches wide, the space between the shell and the tubes is typically only about 4.5 inches wide, and the space between the bottom of the wrapper barrel and the tube sheet is usually only about twelve to fourteen inches high. The bundle of tubes supported by the tube sheet also are very closely spaced together. Therefore, it is difficult to insert a device into the steam generator annulus. Furthermore, it is difficult to pick up objects between the tubes on the tube sheet.
U.S. Pat. No. 4,638,667 discloses a probe positioning apparatus which includes an elongated extensible boom and an elongated flexible probe carrier tape. The boom has an end which is adapted for extension into and retraction from the tube lane of a steam generator. The carrier tape has sprocket holes for transporting the probe. The end of the boom, which is located within the steam generator, has a tractor feed unit and two sprocket belts for feeding the carrier tape. A drive motor rotates the tractor feed unit 180.degree. in order to investigate tube rows on both sides of the tube lane. Located outside of the steam generator is a tape withdrawing assembly which includes a drive motor and a sprocket for retrieving the carrier tape.
U.S. Pat. No. 4,760,876 discloses a transport system for an inspection system having an end effector which carries inspection and gripping devices, a hollow flexible conduit which passes control cables for the end effector therethrough, and a drive assembly which moves the flexible conduit in axial and rotational directions.
U.S. Pat. No. 4,702,878 discloses a device for searching and retrieving foreign objects on the tube sheet of a steam generator. The device includes a probe for searching for foreign objects on the surface of the tube sheet and a gripper for grasping one of the foreign objects. The device is inserted into the annulus using a guide tube having a straight main segment, an L-shaped upper segment which is bent at a right angle with respect to the main segment, and an L-shaped lower segment. The lower segment, which rests on the surface of the tube sheet, is bent at a right angle with respect to the main segment and extends in a direction rotated 90.degree. from the direction of the upper segment. The upper segment is secured to the handhole by a guide plate. To facilitate insertion of the guide tube into the steam generator, the upper, main and lower segments are constructed from separate parts.
U.S. Statutory Invention Registration No. H1115 discloses a robot arm apparatus having two or more cascaded conduit elements and a flexible movable conduit. The flexible conduit passes through the cascaded conduit elements and conveys inspection and/or maintenance apparatus to the interior of a steam generator. The flexible conduit has a terminal working end which is translated into and around the interior of the steam generator. A first reversible motor translates the cascaded conduit elements in a first axis within the tube lane. A second reversible motor translates the flexible conduit in a direction perpendicular to the first axis. A third reversible motor rotates a carriage which carries the cascaded conduit elements and, thus, moves the terminal working end in a third direction of travel, which is a curved path.
It is known to manually utilize a pair of parallel guide tubes between one handhole and the tube lane in order to manually and independently position, through each of the parallel guide tubes, a search probe and a retrieval device. Each guide tube is formed from a plurality of swage-locked conduits and is taped to a corresponding guide tube of the pair. Such parallel guide tubes have an L-bend in order to manually and independently position, through each of the parallel guide tubes, a search probe and a retrieval device in an axis perpendicular to the tube lane. It is also known to manually utilize two guide tubes between one handhole and the annulus. Each guide tube is a flexible conduit and is independent from the other of the two guide tubes. These two guide tubes are used to manually and independently position a search probe and a retrieval device on the tube sheet within the annulus.
In these manners, for example, the search probe may be used to view a foreign object lodged within the steam generator from a variety of angles while the retrieval device has grasped, but not yet retrieved, the object. Also, because of such independent operation, the search probe may be used to search for a second foreign object while the retrieval device retrieves the first foreign object. However, during such manual positioning, inspection personnel for nuclear steam generators are exposed to increased levels of radiation during manual search and retrieval at the handhole.
There is a need, therefore, for an apparatus for searching and retrieving foreign objects from a steam generator which permits inspection personnel to remotely and independently position inspection probes and retrieval devices within a steam generator.
There is a more particular need for such an apparatus which permits inspection personnel to remotely and independently position such probes and devices within the annulus of the steam generator.
There is an even more particular need for such an apparatus which permits inspection personnel to remotely, independently and selectively position such probes and devices within a selected half of the annulus.